1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device using a variable resistive element and a method for driving the same, and more particularly to a method of a forming process for a variable resistive element, as an initializing process.
2. Description of the Related Art
Recently, new type non-volatile semiconductor memory devices have been widely studied as an alternative to a flash memory. Among them, a RRAM (Resistance Random Access Memory) uses a phenomenon in which resistance is changed by applying a voltage to a variable resistor formed of a transition metal oxide and has an advantage in a view of a miniaturization limit compared to the flash memory, and its research and development has been actively undertaken because data can be written at high speed.
The variable resistive element used in the RRAM as the new memory has a structure in which a lower electrode (first electrode), a variable resistor, and an upper electrode (second electrode) are laminated in this order, and has properties in which its resistance value can be reversibly changed by applying an electric stress such as a voltage pulse to between the upper electrode and the lower electrode. The RRAM is the new memory realized by reading the resistance value changed due to this reversible resistance changing action (hereinafter, referred to as the “switching action” occasionally).
As a material of the variable resistor, it is known that the switching action is generated in various kinds of metal oxides. Especially, the phenomenon of the switching action is reported in detail in “Low Power and High Speed Switching of Ti-doped NiO ReRAM under the Unipolar Voltage Source of less than 3 V”, IEDM Technical Digest. 2007, p. 767-770 (hereinafter, referred to as the “publicly known document 1”) by K. Tsunoda et al, as for a nickel oxide (NiO) film, and in “Low Power and High Speed Bipolar Switching with A Thin Reactive Ti Buffer Layer in Robust HfO2 Based RRAM”, IEDM Technical Digest. 2008, p. 297-300 (hereinafter, referred to as the “publicly known document 2”) by H. Y. Lee et al. as for a hafnium oxide (HfO2) film.
However, as for the variable resistive element using the transition metal oxide as the variable resistor, as reported in the publicly known documents 1 and 2, there is a problem that just after the structure in which the variable resistor material is sandwiched between the upper and lower electrodes is produced, its resistance state is higher than a high resistance state provided in the switching action, and the resistance state is not changed. That is, it is known that in order to lower the resistance of the variable resistive element and change the variable resistive element into a state in which the switching action can be performed (variable resistance state), it is necessary to perform a step (hereinafter, referred to as the “forming process” occasionally) of applying a specific electric stimulus to between the upper and lower electrodes.
In other words, the variable resistive element using the metal oxide as the variable resistor is in an insulating state as an initial state just after produced, so that in order to change the variable resistive element into a state in which the resistance state can be switched between a high resistance state and a low resistance state by an electric stress, it is necessary to form a region (hereinafter, referred to as the “filament path” occasionally) in which a resistance rate is locally lowered in the oxide by applying a voltage to this, as shown in the publicly known documents 1 and 2. It is considered that the resistance state is switched because the filament path is formed or cut.
According to the publicly known documents 1 and 2, DC sweep is used in performing the forming process for the variable resistive element (hereinafter, referred to as the “DC forming process”). That is, the resistance of the variable resistive element is lowered while a voltage applied to the variable resistive element is gradually raised to a predetermined voltage to perform the forming process. Therefore, it is likely that it takes a very long time. Since it is expected that hundreds of M to several G-bit memory cells are mounted in an actual RRAM, it needs an extremely long time to perform the forming process. Since the method of the DC forming process is not a realistic way, it is assumed that the forming process is performed by applying a rectangular voltage pulse of several tens of ns to several hundreds of us in general (hereinafter, referred to as the “pulse forming process”).
The inventors of the present invention have closely studied the switching action of the variable resistive element, using the pulse forming process. As a result, it has been found that a writing current required to realize the stable switching action differs between the DC forming process and the pulse forming process, and the several-fold writing current is needed in the pulse forming process.
Various factors are considered for this reason, but it is estimated that an electric property or structural property of the filament path formed by the forming process differs between the DC forming process and the pulse forming process.
A large writing current causes various kinds of disadvantages. For example, in order to implement the normal switching action, a selection transistor having a high current driving ability is needed, and the selection transistor becomes large in size, so that the memory cell becomes large, and it is extremely difficult to implement a large-capacity memory. Thus, this is not suitable for dealing with large data such as a movie file which is expected to be increasingly demanded in the future.
Furthermore, when the writing current needed is large, a surrounding circuit such as a writing voltage application circuit becomes large, and accordingly a chip size becomes large, so that it is difficult to provide a non-volatile memory at low cost. In addition, power consumption increases as a matter of course, so that it is considerably difficult to be mounted on a mobile device such as a mobile terminal which requires low power consumption.